Known variable speed V-belts for automotive four-wheel vehicles, motorcycles and the like include a single cogged V-belt with cogs on the belt inner face only, a double cogged V-belt with cogs on both the belt inner and back faces, and a hybrid V-belt in which a plurality of blocks are engaged at specified pitches and intervals in the belt lengthwise direction with bilaterally paired tension members. For heavy duty power transmission V-belts, lateral pressures from pulleys are high. Therefore, in order to withstand such high lateral pressures, rubber compositions used for the above various kinds of variable speed heavy duty power transmission belts are required to have a high elastic modulus after crosslinked. In addition, these rubber compositions after crosslinked are also required to have excellent heat aging resistance and flex fatigue cracking resistance and exhibit a small permanent set due to temperature and pressure.
To meet these requirements, use has conventionally been made of peroxide-crosslinked hydrogenated acrylonitrile butadiene rubbers reinforced with a metal salt monomer of an unsaturated carboxylic acid, typically, such as zinc dimethacrylate or zinc diacrylate. For such hydrogenated acrylonitrile butadiene rubbers (HNBR), as the content of metal salt monomer is increased, their elastic modulus becomes higher and therefore they become more advantageous to heavy duty power transmission. However, on the other side of the coin, the HNBRs deteriorate their heat aging resistance and flex fatigue cracking resistance, thereby tending to increase permanent set. To cope with this, a technique has conventionally been employed which determines an optimal amount of the unsaturated metal salt monomer mixed into a rubber composition for a belt in consideration of the balance between belt power transmission capability required according to the particular application, and required heat resistance, cracking resistance and low permanent deformability.
Meanwhile, there has been increasing demand in recent years for development of power transmission belts which can satisfy not only the foregoing characteristic requirements but also their durability requirement even when used in cold climates such as North America and North Europe, or in other words, globally serviceable power transmission belts.
If the above-mentioned HNBR-based rubber compositions are used to meet, for example, flexibility and softness requirement at −35° C., a large amount of plasticizer such as oil needs to be mixed thereinto. Then, the resultant rubbers would have adverse effects of decreased elastic modulus and increased permanent set on belts using the rubbers, and in turn the belts would be difficult to preserve the above good balance among the foregoing characteristics.
On the other hand, attention has in recent years been given, as rubber materials for power transmission belts having both heat resistance and cold resistance, to ethylene-α-olefin elastomers such as ethylene-propylene copolymer (EPM), ethylene-propylene-diene terpolymer (EPDM) or ethylene-octene copolymer.
For ethylene-α-olefin elastomer-based rubber compositions, however, the resultant rubbers have a low elastic modulus. It may be possible to enhance the elastic modulus of such rubber by increasing the amount of filler such as carbon black. In that case, however, there arise a problem that a belt using the rubber exhibits an extremely large self-heating when bent and has a poor cracking resistance, thereby not reaching a practical level. To solve this problem, it has been considered to enhance the rubber elastic modulus by adding a metal salt monomer of an unsaturated carboxylic acid to the ethylene-α-olefin elastomer.
For example, Japanese Unexamined Patent Publication (Kokai) No. 5-17635 describes a belt rubber composition in which 100 parts by weight of an ethylene-α-olefin elastomer having a Mooney viscosity (ML1+4, 100° C.) of 10 to 70 and containing 75 mol % or more of ethylene units is mixed with 15 to 80 parts by weight of a metal salt monomer of an ethylene-unsaturated carboxylic acid and 0.2 to 10 parts by weight of an organic peroxide.
International Patent Publication Number WO97/22662 describes a belt rubber composition in which 100 parts by weight of an ethylene-α-olefin elastomer having an ethylene content of 40 to 70 wt % is mixed with 32 to 100 parts by weight of a metal salt of an unsaturated carboxylic acid.
Japanese Unexamined Patent Publication (Kohyo) No. 9-500930 and its corresponding U.S. Pat. No. 5,610,217 describe belt rubber compositions in which 100 parts by weight of an ethylene-α-olefin elastomer is mixed with 1 to 30 parts by weight of a metal salt of an α-β-unsaturated organic acid and 250 parts by weight or less of a reinforcing filler. These documents also describe a technique that up to 25 parts by weight of HNBR is blended into 100 parts by weight of the ethylene-α-olefin elastomer.
International Patent Publication Number WO97/22663 describes a belt rubber composition in which a basic rubber blend composed of 41 to 49 parts by weight of an ethylene-α-olefin elastomer and 51 to 59 parts by weight of HNBR is mixed with 5 to 80.5 parts by weight of a metal salt of an unsaturated carboxylic acid.
However, in the case of adding a metal salt monomer of an unsaturated carboxylic acid to an ethylene-α-olefin elastomer, the resultant rubber has a high elastic modulus but extremely deteriorates its cracking resistance. The above-described Japanese Unexamined Patent Publication (Kokai) No. 5-17635 mentions that a rubber composition in which an ethylene-α-olefin elastomer having an ethylene content of about 70 mol % or less is mixed with a metal salt monomer of an unsaturated carboxylic acid would not provide high strength, and presumes that this is due to poor dispersibility of the metal salt monomer. The above-described problem of deteriorated cracking resistance is presumably also due to poor dispersibility of the metal salt monomer. In the above case of using the ethylene-α-olefin elastomer containing 75 mol % or more of ethylene units, the resultant rubber achieves a high strength, but causes crystallization at low temperatures because of its high ethylene content. Therefore, it does not go far enough to satisfy the flex resistance of a belt at low temperatures.
The above technique of blending an ethylene-α-olefin elastomer and HNBR may be effective in enhancing the cold resistance of the resultant rubber. However, since the content of ethylene-α-olefin elastomer is as small as 41 to 49 parts by weight in 100 parts by weight of the entire rubber component, this does not have a sufficient effect of enhancing the rubber cold resistance. On the other hand, if the amount of oil mixed is increased in order to improve the cold resistance, this easily causes a permanent deformation (permanent set) of the rubber.
Furthermore, in the above case of a blending ratio of 25 parts by weight or less of HNBR to 100 parts by weight of ethylene-α-olefin elastomer, its high blending ratio of ethylene-α-olefin elastomer increases the cold resistance but deteriorates the cracking resistance, which is disadvantageous to durability at high temperatures.
The present invention is directed to the fabrication of a heavy duty power transmission belt such as a variable speed belt using a rubber composition obtained by blending ethylene-α-olefin elastomer and HNBR, and has an object of achieving a good balance among high elastic modulus, heat aging resistance, cracking resistance, permanent set resistance and cold resistance.
Furthermore, the present invention has another object of enhancing the cold resistance of the belt without impairing its durability at high temperatures, ensuring its sufficient power transmission capability and concurrently enhancing its permanent set resistance.